In recent years, more commercial thermoplastic rubber products have appeared in bikes, gifts, toys and the medical industry. Soft, stretchable, clear, transparent products in the area of amusement, imaginative play, and ornamentations are particularly attractive to consumers. However, thermoplastic rubber compounds containing mineral oil do not provide a good surface to achieve permanent marking by conventional printing methods such as pad printing, screen printing, or hot foiling. A soft thermoplastic rubber compound with hardness below 10 A makes it even more difficult. If the ink film's elasticity is lower than that of the substrate, the printed image will separate when stretched. In the prior art U.S. Pat. No. 3,519,466 to Akamatsu, et al. discloses a process for printing on molded articles of a thermoplastic resin or rubber. This process involves heating an ink containing a benzene-soluble reactive dye and a wetting agent soluble in benzene in contact with the surface of the molded article which contains an organo-metallic compound. When the article reaches a temperature higher than 50° C. but lower than the softening point of the article, the dye permeates the article and reacts with the organo-metallic compound to become fixed therein.
Then U.S. Pat. No. 6,367,384 to Cass, Donald W. reveals a process for printing a four color image directly onto a fishing lure. The process begins by preparing the soft bait fishing lure to receive the ink. By employing a four color pad printing machine the process prints a four color image on one side of the fishing lure and after optionally turning the lure over, prints on the other side. In the Cass process, all of the printing articles must be pretreated and a clear topcoat may optionally be applied to the lure.
The current available methods to apply images onto thermoplastic rubber compounds include hand painting or spraying. Since the composition of thermoplastic rubber compounds contains from about 60% to about 80% mineral oil by weight, ink film has extreme difficulty sticking to the surface of the substrate. The present invention solves these problems by using the pad printing process to apply an image to the surface of thermoplastic rubber compounds directly and to apply a topcoat that protects the applied image even when stretched.